The partial oxidation of hydrocarbons, for example methane or natural gas, in the presence of a catalyst is an attractive route for the preparation of mixtures of carbon monoxide and hydrogen, known in the art as synthesis gas. The partial oxidation of a hydrocarbon is a highly exothermic reaction and, in the case in which methane is the hydrocarbon, proceeds by the following reaction: EQU 2CH.sub.4 +O.sub.2 .fwdarw.2CO+4H.sub.2
A number of process regimes have been proposed in the art for carrying out the catalytic partial oxidation reactions. One regime that is most suitable for application on a commercial scale is to contact the feed gases with the catalysts retained in a fixed arrangement, for example as a fixed bed of particles or a monolith structure. The literature contains a number of documents disclosing details of experiments conducted into the catalytic partial oxidation of hydrocarbons, in particular methane, employing a wide range of catalysts in a fixed bed arrangement.
European Patent Application publication No. 0 303 438 (EP-A-0 303 438) discloses a process for the catalytic partial oxidation of a hydrocarbonaceous feedstock in which a gaseous mixture of the hydrocarbonaceous feedstock, oxygen or an oxygen-containing gas and, optionally, steam, is introduced into a catalytic partial oxidation zone to contact a catalyst retained therein. The catalyst employed in the process may comprise a wide range of catalytically active components, for example palladium, platinum, rhodium, iridium, osmium, ruthenium, nickel, chromium, cobalt, cerium, lanthanum and mixtures thereof. Further, it is stated in EP-A-0 303 438 that materials not normally considered to be catalytically active may also be employed as catalysts, for example refractory oxides such as cordierite, mullite, mullite aluminium titanate, zirconia spinels and alumina. The catalyst may be of a variety of forms, for example sheets of corrugated metal packed to form elongate channels therethrough or wire mesh. However, preference is given in EP-A-0 303 438 to the use of catalysts in the form of extruded honeycomb monoliths. These monoliths comprise a large number of parallel channels extending through the structure in the direction of flow of the feed and product gasses.
European Patent No. 0 262 947 (EP-B-0 262 947) discloses a process for generating hydrogen by the partial oxidation of a hydrocarbon in which a mixture of the hydrocarbon and oxygen is injected into a mass of a catalyst. The catalyst disclosed in EP-B-0 262 947 comprises platinum and chromium oxide supported on a refractory solid. The support structures described in EP-B-0 262 947 are honeycomb monolith supports, of the type used in purifying the exhausts from motor vehicles or from chemical plants, and particulate supports, preferably comprising particles having a maximum dimension of from 1 to 4 mm, for example 1.5 mm.
D. A. Hickman and L. D. Schmidt ("Synthesis Gas Formation by Direct Oxidation of Methane over Pt Monoliths", Journal of Catalysis 138, 267-282, 1992)) have conducted experiments into the partial oxidation of methane in the presence of catalysts comprising either platinum or rhodium. The partial oxidation reactions were conducted at substantially atmospheric pressure and at temperatures in the range of from 600 to 1500 K (337.degree. to 1237.degree. C.). The catalysts employed were in the form of metal gauzes, metal-coated foam monoliths and metal coated extruded monoliths. The metal gauze catalysts comprised 1 to 10 layers of gauzes of either 40 mesh (40 wires per inch) or 80 mesh. The foam monoliths were of alpha-alumina and described as having an open cellular, sponge-like structure. The samples employed had a nominal porosity of 30 to 50 pores per inch (ppi). The extruded monoliths were cordierite extruded monoliths, having 400 square cells/in.sup.2 and consisted of straight parallel channels giving laminar flows of gases through the channels under the conditions of gas flow rate studied.
J. K. Hockmuth ("Catalytic Partial Oxidation of Methane over a monolith Supported Catalyst", Applied Catalysis B: Environmental, 1 (1992) 89-100) reports the catalytic partial oxidation of methane using a catalyst comprising a combination of platinum and palladium supported on a cordierite monolith body.
A number of academic experiments have been reported in the literature in which catalysts have been employed in the form of fixed beds of catalyst particles.
Thus, A. T Ashcroft et al. ("Selective oxidation of methane to synthesis gas using transition metal catalysts", Nature, vol. 344, No. 6264, pages 319 to 321, 22nd March, 1990) disclose the partial oxidation of methane to synthesis gas in the presence of a range of ruthenium-containing catalysts. The objective of the experiments was to establish that the partial oxidation process could be carried out under mild conditions and at low temperatures. To this end, the experiments were conducted with a low gas hourly space velocity of 40,000/hr, a pressure of 1 atmosphere and a temperature of about 775.degree. C. The catalyst employed comprised small amounts of a solid, powdered catalyst.
P. D. F. Vernon et al. ("Partial Oxidation of methane to Synthesis Gas", Catalysis Letters 6 (1990) 181-186) disclose a range of experiments in which catalysts comprising nickel, ruthenium, rhodium, palladium, iridium or platinum, either supported on alumina or present in mixed oxide precursors, were applied. Again, the experiments reported are limited to a catalytic partial oxidation process employing only mild operating conditions and using small amounts of catalyst in the form of pellets retained in a fixed bed. The authors report the same experiments in "Partial Oxidation of Methane to Synthesis Gas, and Carbon Dioxide as an Oxidizing Agent for Methane Conversion", Catalysis Today, 13 (1992) 417-426.
R. H. Jones et al. ("Catalytic Conversion of Methane to Synthesis Gas over Europium Iridate, Eu.sub.2 Ir.sub.2 O.sub.7 ", Catalysis Letters 8 (1991) 169-174) report the selective partial oxidation of methane using the europium iridium pyrochlore Eu.sub.2 Ir.sub.2 O.sub.7. The reaction was studied under the mild conditions of a pressure of 1 atmosphere and a temperature of 873 K (600.degree. C.). The catalyst was prepared by grinding and subsequent pressing to form pellets. The pelletized catalyst was packed into a porous silica frit and used directly in the experiments.
U.S. Pat. No. 5,149,464 is directed to a method for selectively oxygenating methane to carbon monoxide and hydrogen by bringing the reactant gas mixture at a temperature of about 650.degree. C. to 900.degree. C. into contact with a solid catalyst which is generally described as being either:
a) a catalyst of the formula M.sub.x M'.sub.y O.sub.z, where:
M is at least one element selected from Mg, B, Al, Ln, Ga, Si, Ti, Zr and Hf; Ln is at least one member of lanthanum and the lanthanide series of elements; PA1 M' is a d-block transition metal, and each of the ratios x/y and y/z and (x+y)/z is independently from 0.1 to 8; or
b) an oxide of a d-block transition metal; or
c) a d-block transition metal on a refractory support; or
d) a catalyst formed by heating a) or b) under the conditions of the reaction or under non-oxidizing conditions.
The d-block transition metals are said in U.S. Pat. No. 5,149,464 to be selected from those having atomic number 21 to 29, 40 to 47 and 72 to 79, the metals scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, palladium, silver, hafnium, tantalum, tungsten, rhenium, osmium, iridium, platinum and gold. It is stated in U.S. Pat. No. 5,149,464 that the preferred metals are those in Group VIII of the Periodic Table of the Elements, that is iron, osmium, cobalt, rhenium, iridium, palladium platinum, nickel and ruthenium.
The process described in U.S. Pat. No. 5,149,464 is operated at a temperature in the range of from 650.degree. C. to 900.degree. C., with a range of from 700.degree. C. to 800.degree. C. being preferred. A range of experiments are described in U.S. Pat. No. 5,149,464 in which a variety of catalysts comprising Group VIII metals were tested, including ruthenium oxide, presidium/ruthenium oxides, pyrochlores, ruthenium on alumina, rhodium on alumina, palladium on alumina, platinum on alumina, nickel/aluminium oxide, perovskites and nickel oxide.
A similar general disclosure of a catalyst for use in the catalytic partial oxidation process is made in International Patent Application publication No. WO 92/11199. WO 92/11199 specifically discloses experiments in which catalysts comprising iridium, palladium, ruthenium, rhodium, nickel and platinum supported on alumina were applied. All the experiments were conducted under mild process conditions, with typical conditions being a pressure of 1 atmosphere, a temperature of 1050 K (777.degree. C.) and a gas hourly space velocity of about 20,000/hr.
The experiments described in both U.S. Pat. No. 5,149,464 and WO 92/11199 employed catalysts in the form of solid powdered particles retained in a fixed bed arrangement by packing in a reaction tube between two plugs of silica wool.
For successful operation on a commercial scale, the catalytic partial oxidation process must be able to achieve a high conversion of the hydrocarbon feedstock at a high gas hourly space velocities. Further, the selectivity of the process to the desired products of carbon monoxide and hydrogen must be high. Both these factors must be met using process equipment which is both economical to construct and economical to operate. In this respect, there exists a significant problem in operating the catalytic partial oxidation process with a fixed bed of catalyst, in that the pressure drop encountered when using the fixed bed prevents the process operating under the high gas space velocities demanded of a commercial operation.
Surprisingly, it has now been found that catalytic partial oxidation can be significantly improved if the process is operated with the catalyst retained in a fixed bed arrangement meeting a very specific set of criteria. In particular, it has been found that the selectivity of the process is significantly improved if the fixed bed arrangement combines a high tortuosity with a high number of pores. Specifically, it has been found that the selectivity is significantly improved if the fixed arrangement has a tortuosity of greater than 1.1 and at least 750 pores per square centimeter. Further, it has been found that using a fixed arrangement meeting these two criteria allows the amount of catalytically active metal present in the catalyst to be reduced whilst still maintaining a high level of activity and selectivity.